During oil and gas production, and in particular during work-over or testing wells, wire line operations are common. Strings, for instance cables, lines, wires, coils or piano wires passes through the wellhead for supporting and manipulating various tools and instruments.
Ideally, the string is removed from the well before the gate valve is closing. However, in some situations, string removal prior to valve closure is not possible and the gate valve is closed on the string. Cutting valves having a wire passing through it should be capable of shearing the string while closing and maintaining a seal after the valve is seated.
It is important that the shearing efficiency of the cutting device is high in order to minimize the risk of dragging in wire strands between the upper seat and the gate. This can cause a leaky seal in the valve if the wire or strands is not sheared or cut off properly.
There has to be a clearance between the block and the seats to allow thermal expansion of the gate. While the valve is closing, the upper and lower seat retracts into a respective valve seat pocket. In prior art the cutting valve is symmetric about a centerline, this means that the seats, resilient members etc are equal at the upper side of the gate and at the lower side of the gate.
The weight of the gate is typically between 50-120 kg, gravity will pull the gate downward, and this may lead to large deviations between the centerline of the gate and the centerline of the cavity of the cutting valve and thus also a larger clearance between the upper seat and the valve block than the lower seat and the valve block. In the cutting position the upper seat is thus allowed to retract more than the lower seat, causing a larger clearance between the upper seat and the gate than between the lower seat and the gate. This is further explained below.
Typically braided wires with tough and thin strands, like coils, piano wires or other strings, consisting of a bundle of strands, do particularly represents a risk of dragging in parts of the strands between the clearance of the gate and the seats, causing a leaky seal.
It is inconvenient and difficult to change the cutting valve in the well, it is therefore necessary to install a cutting valve that can cut different types of objects such as cables, wires, strings, coils, piano wires etc in the well, efficiently.
US 20130119288 discloses a prior art cutting valve capable of shearing through any downhole tooling and maintain its seal capability after the shearing operation is complete. The cutting valve comprises a gate that is slidable within a cavity in the valve block from an open position to a closing and cutting position. The valve having seats mounted on opposite sides of the gate and having sealing surfaces that engage the gate when the gate is in the closed position. Shear elements are connected to and carried by each of the valve seats and are capable of axial movement relative the valve seats in response to movement of the gate from the open position to the closed position.
The seats and shearing arrangement on the upper and lower side of the gate are described to operate in the same manner. The publication does not take into consideration that the gate have a considerably weight will pull the gate downward in the cutting process.
There are not described any adjustment caused by the gravity of the gate valve in the publication. This indicates that it is not obvious for a skilled person to compensate for the forces acting on the upper and lower seat so that the allowed travelling distance for the upper seat and lower seat are substantially equal.
The arrangement of the shear elements in the publication are also more complex than the cutting elements in the present invention. In the present invention the cutting elements forms an integrated part of the gate.
A demand exists for a cutting valve which minimizes the clearance between the upper seat and the gate in the cutting position, and which increase the shearing efficiency of the valve and thereby ensures that the wire is cut off, i.e. severed, completely.